Progressive Collapse

Domino, Pancake, Zipper, ... Choose Your Metaphor

A feature of the Twin Tower collapses that needed some explaining
was how they pulverized themselves from the crash zones,
staying centered all the way to the ground.
Why didn't the tops topple, like any other vertical
structure would topple from the fulcrum of the damaged area?
Enter progressive collapse,
a theory so versatile it applies to all of the
accepted collapse theories.
"Progressive collapse" would be the umbrella giving a legitimate sound
to the domino, pancake, and zipper metaphors used by
educators such as
Thomas Eagar.

Illustration in the September 12, 2001 edition of the New York Times

Typing progressive collapse into a search engine
reveals that structural engineers are
"discovering new ways to deal with this deadly phenomenon"
1Â
If one researches the history of total progressive collapse
of buildings one will find that,
in regard to the structural failure of large
steel-framed or steel-reinforced-concrete-framed buildings
it is a recent phenomenon,
apparently starting with the Oklahoma City Bombing,
and then rearing its ugly head again in the 9/11/01 attack.

The Ronan Point apartment building following a progressive collapse
of cantilever balcony structures

Outside of these events, the most commonly cited example of
progressive collapse is the collapse of one corner of
a 23-story block of flats in Newham in east London,
known as Ronan Point.
At 5:45 AM on May 16, 1968, an explosion in a corner flat
of the building's 18th floor blew out sections of the outer wall,
causing a chain-reaction collapse of a stack of floor and wall sections
on the building's southeast corner.
2Â

Any comparisons of the Ronan Point incident
to the collapses of the Murrah Federal Building
and the World Trade Center skyscrapers
are constrained by the fact
that the section of the Ronan Point building that collapsed
was not part of the support structure of the building.
Rather, the collapsed balconies were short cantilever sections
supported by the building's main structure.
This contrasts with the collapse of a large structural section
of the Murrah Building,
and the total collapses of the World Trade Center skyscrapers.

The problem with the progressive collapse theory
is that it's very difficult to actually build something
that will exhibit this behavior.
To attempt to verify that this phenomenon can be reproduced
outside of a terrorist incident we have introduced the
progressive collapse challenge.